Electrically continuous thin metal layers or films, formed on rigid dielectric substrates by vacuum metallization, have been long used to give substrates a reflective metallic appearance. To slow corrosion of the metal layer, the layer was typically top coated with a clear, colorless dielectric polymeric coating. However, once the top coats are damaged or experience water infiltration, these metal films frequently experienced widespread corrosion of the metal layer.
More recently, electrically discontinuous metal layers have been developed which appear as continuous metal layers to the naked eye, which are less susceptible to widespread corrosion and which can be applied to flexible substrates. These electrically discontinuous layers consist of discrete metallic islands, which are vacuum deposited on the substrate, wherein the islands are separated by channels. These islands and channels are then top coated with a dielectric polymeric coating to separately encapsulate each island and to prevent corrosion of the metal islands. However, under weathering conditions the top coat has experienced water infiltration and/or a loss of adhesion (e.g., peel) from the metal islands and substrate in the channels.
To provide adequate adhesion of the top coat to the discontinuous layer, the metal layer has been etched with a caustic (e.g., sodium hyroxide solution) to remove metal deposited in the channels between the islands to provide a larger substrate surface area for bonding with the top coat. However, caustic etching can result in the formation of blackened areas in the metal layer.
Therefore, a need exists for a means of vacuum metallization of rigid and flexible substrates wherein the top coat will adhere to the metallized layer without etching, ad the side effects of etching, and wherein the top coat is less susceptible to water infiltration and loss of adhesion over time and with weathering.
This invention relates to a metallized article comprising a substrate, preferably having a polyurethane basecoat disposed thereon, with a layer of electrically discrete metallic islands of a corrosion prone metal disposed on the substrate. A crosslinked polyurethane top coat, containing an aminosilane, is further disposed on and encapsulates the discrete metallic islands.
The advantage of this invention is that it improves the bonding of polyurethane top coats to metal islands, deposited in a layer on a substrate, without caustic etching of the metal layer. This invention also has the advantage of increasing the hardness and the water impermiability of the top coat, by polymeric crosslinking, to further improve corrosion resistance.
The substrates of the present invention include any substrate upon which a reflective metallic coating is desirable. These substrates can be rigid or flexible. Further, these substrates and may or may not be electrically conductive.
Typically, substrates used in the present invention include vehicular/automotive trim applications, sheet stock, sports equipment, clothing and any other items suitable for decoration by inclusion of a reflective metallic surface.
Examples of suitable nonconducive (dielectric) substrates include a wide variety of plastic substrates which are dielectric materials (non-conductive) including thermoplastic materials, thermosetting materials and elastomeric materials, such as thermoset polyurethane, flexible elastomers which may be a natural or synthetic thermoplastic or thermoset polymer having an elongation of at least 30%, polyolefins, as polyethylene, polypropylene, polybutylene or a rubber/polypropylene blend, ABS (polyacrylonitrile-butadiene-styrene), thermoplastics as polyvinyl chloride, Surlyn (DuPont), polyester, polyester elastomer, and the like. Articles made of plastic substrates include, for example, automobile parts such as exterior moldings, bumper guards, dual pulls, mirror housings, grill headers, light bezels, gear shift bezels, door pulls, steering wheel emblems, and other exterior and interior automotive trim components. Other plastic articles can be used, for example in the plumbing trade, for household hardware applications, for home decoration, trucks, motor cycles and marine parts.
Examples of suitable conductive substrates include metals, such as aluminum, aluminum alloy, carbon steel, cast iron, brass, copper, nickel, nickel alloy, stainless steel, magnesium alloy and zinc based materials. Articles comprising metal substrates include, for example, faucets, knobs, handles, cutlery, files and blades, golf clubs and irons, hammers, jet blades, rifle barrels, skate blades, camera components and luggage. Preferably, the metal substrate is a vehicle wheel.
It is to be understood with respect to many of the metallic substrates used in the present invention, in particular for wheels, that these substrates may be pretreated prior to the present application process. Such pretreatment may optionally include picketing and/or the application of corrosion resistant coatings. Those corrosion resistant coatings can be phosphate corrosion resistant coatings or epoxy primers such as xe2x80x9cE-coatxe2x80x9d, i.e., a cathodic electrocoat or a coating utilizing powder particles. With respect to aluminum and magnesium alloys, such a corrosion resistant coating may include well known chromium conversion coatings and the like.
It is also understood that an adhesion promoter may be applied to non-metallic substrates, such as chlorinated polyolefin to thermoplastic olefins. Typically, a coating thickness of about 0.1 mils to about 0.4 mils is applied.
The preferred substrates for the present invention are rigid substrates.
The metals that are used to form the layer of metallic islands are metals, or surface oxidized metals that will give a bright surface. Suitable metals are corrosion prone metals including tantalum, copper, silver, nickel, chromium, tin and aluminum and alloys thereof, and the like. Preferably, the metallic islands contain indium, indium alloys and/or indium oxides.
The layer of metallic islands is formed by depositing metal on the substrate, or coated substrate, by thermal evaporization, sputtering, ion plating, induction heating, electron beam evaporization and like methods. More uniform coverage is obtained, particularly around corners, edges or recesses if the metallization occurs in a chamber containing an inert gas such as argon.
The method for forming a layer of metallic islands, on a substrate, a treated substrate or a coated substrate, is described in U.S. Pat. Nos. 4,407,871 and 4,431,711 which are incorporated herein by reference.
Metallization produces a substrate that has a layer of discrete metallic islands deposited thereon. The discrete metallic islands are round in nature and have a thickness, or diameter, small enough to make the metallic film electrically non-conductive, as there are channels between the islands such that there is typically no conductivity between the islands, and alternately large enough to reflect enough light to make the coated article appear as a metal article to the naked eye. Typically, the thickness of the metallic islands will be between 25 and 4000 Angstroms (xc3x85), preferably 500-3000xc3x85. Most preferably, the thickness is between 500 xc3x85-1200 xc3x85.
In the present invention the layer of metallic islands on the substrate is encapsulated by a top coat. Preferably, a prime coat and/or basecoat was also applied to the substrate prior to metallization.
Typically, the coating composition for the prime coat, basecoat and/or top coat, after curing is a polyurethane or a polyester polyurethane. A resin suitable for forming basecoats and top coats useful in the present invention is described in Example 1.
To increase cross-linking of the polymer in the top coat and to at least partially improve adhesion to the metallized substrate, at least one organosilane is added to the top coat resin. A description of the use of organosilanes in resin top coats, for application to metal island layers, is described in U.S. patent application Ser. No. 08/576,072, files Aug. 25, 1996 now abandoned, which is incorporated in its entirety herein by reference.
At least one organosilane must be an organosilane that promotes crosslinking of the urethane in the top coat composition. The organosilane is preferably a secondary aminosilane and more preferably is bis-(gamma-trimethoxysilylpropyl)amine. The organosilane is reacted in the resin from which the base coat and top coat are made.
Preferably, the basecoat also contains the secondary aminosilane.
The purposes of the organosilane are to increase crossliking of the polymer in the top coat, such as by end-capping free isocyanates in urthane prepolymers, to increase the hardness and hydrophobicity of the top coat, and to increase adhesion of the top coat to the metallized substrate.
In the embodiment wherein the organosilane is a secondary aminosilane, the ratio of aminosilane to free isocyanate in the coating utilized is typically between 1:1 to 1.2:1. Preferably, the aminosilane is added in excess, typically with a ratio of aminosilane to free isocyanate of between about 1.05:1 to 1.2:1. A suitable top coat containing an aminosilane is further described in Example 3.
In an alternate embodiment, the topcoat contains both an epoxy silane, such as gamma-glycidoxypropyltrimethoxy silane and a secondary aminosilane, such as bis-(gamma-trimethoxysilylpropyl)amine. For rigid substrates, the ratio of epoxy silane to aminosilane in the coating is typically between 1:20 to 1:5. Preferably, the ratio is 1:10.
For flexible substrates, the ratio of expoy silane to aminosilane in the topcoat is typically between 20:1 to about 5:1. Preferably, the ratio is 10:1.
The coating compositions, whether they be base coat and/or top coat is cured at a temperature that is high enough to completely cure the coating material but low enough such that the coating does not burn or significantly discolor. Typically, the coating is cured at a temperature range of approximately 150-375xc2x0 F. for a period of time of 10 minutes to 70 minutes. The coating is preferably cured at a temperature between 250xc2x0 F. to 300xc2x0 F.
The thickness of the coating is typically between 1 mil to 5 mils. Preferably, the coating thickness is between 1.5 mils to 2.5 mils.
The method for applying a prime coat, basecoat, combined primer/basecoat or top coat composition, to a substrate or a layer of metallic islands, is described in U.S. Pat. Nos. 4,407,871, 4,431,711 and 5,468,518 which are incorporated herein by reference. Typical methods include spray coating, dip coating, flow coating and knife-over-roll coating.
Generally, a coating is applied in an organic solvent system wherein the organic solvent(s) comprise 40% to about 90% of the weight of the pre-cured coating composition. The urethane resin is typically 10% to 50% by weight of the pre-cured coating composition.
A wide variety of organic solvents can be utilized for the commercially available coating compositions, such as aromatic hydrocarbons, alkylesters, alcohols, ketones and dialkylethers. Preferably, the organic solvent is a solvent blend as is described in Examples 2 and 3.
The application of the coating system described herein is preferably performed by an airless spray gun. The coatings are applied to the substrate at ambient temperature and pressure.
In the application of the coating system to the substrate whether as a basecoat, primer coat or top coat, inorganic carriers, such as carbon dioxide, can be substituted for a portion or all of the organic solvent carriers. The method for applying a coating with a reduced amount of organic solvent is described in U.S. Pat. No. 5,464,661 which is incorporated herein by reference.
The Unicarb(copyright) System (Union Carbide) is a useful apparatus for replacing liquid organic solvent with CO2 in spraying coatings in the present invention.
In the method of the present invention, the coatings are typically flashed for approximately 10 to 20 minutes to evaporate the solvents in the coating system and optionally by a curing step after application of each layer. Alternatively, it may be desired to apply another coating after the flashing of the solvent flashing has occurred. This can be characterized as a wet-on-wet system. All that is required after the first coating that is applied, that it is not fully cured. The substrate is in a handleable or tacky condition, prior to application of metal.
Optionally, additional amounts of pigment may be added for a prime or a basecoat in the amount of 0.1% to 40% by weight of the pre-cured (e.g., sprayable) coating composition. Preferably, the amount of pigment is between 20% to 30% by weight.
The invention will not be further and specifically described by the following examples.